Add backwards compatiblility for Rails Tutorial

The Rails Tutorial is unusual in that it is not only pure PolyTeX,
but it also uses a raw <book name>.tex in place of Book.txt. This
story handles backwards-compatibility for such books using the new
cleaned-up directory structure.

[Delivers #62992896]
Issue pull request for branch working-with-rt-62992896

[Delivers #62992896](https://www.pivotaltracker.com/story/show/62992896)
Merge branch 'master' into working-with-rt-62992896
Merge pull request #82 from softcover/working-with-rt-62992896

Working with rt 62992896